The invention relates to the field of gene technology and is concerned with recombinant DNA molecules, which contain a nucleotide sequence coding for a protein or polypeptide of the same IgG specificity as protein G from group G Streptococci strain G148. Moreover the invention comprises micro-organisms containing the aforesaid molecules, and the use thereof in the production of the aforesaid protein or polypeptide.
The existence of bacterially produced proteins or poly-peptides binding selectively to the Fc portion of immunoglobulins has been known for decades. Among such proteins the one that has been investigated most thoroughly is protein A from Staphylococcus aureus; and this is actually the only protein of that group which has acquired commercial importance (Protein A and Protein A Sepharose.RTM. from Pharmacia AB, Uppsala, Sweden). Sjoquist (FEBS Letters 28(1) (1972) p. 73-76 and U.S. Pat. Nos. 3,850,798 and 3,995,018) made the discovery that one component of the protein A-IgG system when immobilized on an insoluble polymer could be employed for binding the other component, and it became thus possible to purify the protein on a large scale and, also, to develop a very simple and selective method for the isolation of antibodies. The specificity of the protein for various different immunoglobulins has been studied in great detail; see for instance Forsgren, A. et al., Staphylococci and Staphylococcal Infections 2 (1983), p. 429-479, Academic Press Inc. (London). A remarkable feature of that protein is its low affinity for the IgG subclass IgG.sub.3 of the human system. In view of the fact that this subclass amounts to about 8% of the total IgG content in serum supplementary purification steps are required for isolating all classes of IgG.
However, already in 1973 Kronvall (J. Immunol. 111(5) (1973) p. 1401-6) showed that also group C and G Streptococci carry components on their surface which have affinity for IgG. The experiments furthermore showed that their reactivity in this respect was a non-immunoreactivity, that is, the immunoglobulin was not bound via its antigen combining sites in the Fab portions, and the results indicated that IgG.sub.3 too was bound in this system. This Fc-binding component has later on been found to be of a protein nature and has been given the name of "protein G". Therefore, as a supplement to the Staphylococcus aureus protein A or as a total or partial replacement thereof, protein G would be a natural choice among bacterially produced proteins capable of binding to the Fc portion of IgG class immunoglobulins, for example for the purpose of purifying IgG (U.S. Pat. No. 3,995,018). But the protein has not been studied to any major extent, due to problems involved with its liberation from the bacterium.
In Journal of Immunol. 133(2) p. 969 Bjorck and Kronvall in 1984 published a method by which small amounts of the protein were set free from the bacterium by means of enzymatic degradation with papain. This method will give a material enabling limited functional and structural studies to be carried out, but the yields are far too low for commercial use, e.g. for the purification of monoclonal antibodies. Quite generally also it may be difficult to obtain a homogeneous and reproducible product with such a method. Moreover Streptococci are pathogenic and necessitate the use of complex culturing media which involve complications in large-scale cultures. There is thus a need for a new method of producing Fc-binding proteins or fragments thereof from Streptococci of types C and G.